This invention belongs to the field of thermoplastic resins. In particular, it relates to nanocomposite compositions comprising plasticizers and their use in polymer processing.
The most common means of producing a finished article, such as a film or injection molded part, from a plasticized thermoplastic resin consists of the following steps: (1) mixing the molten thermoplastic with the plasticizer or plasticizers (2) isolating this mixture as a solid, usually in the form of pellets and (3) supplying this solid mixture to a second thermal process, such as extrusion or injection molding, in order to convert the plasticized thermoplastic material into the finished article. Such a process is described in U.S. Pat. No. 4,889,673, U.S. Pat. No. 5,362,778, and in European Patent Application Publication No. 0413972A2.
U.S. Pat. No. 3,667,733 describes a device that can be placed immediately before the thermal conversion process to produce the finished article; the purpose of this device is to circumvent the need for a separate compounding step. This patent recognized the expense associated with the compounding step but the device claimed in this patent would require considerable capital to build and install. This patent does state, xe2x80x9cthat it is, in effect, impossible to plasticate in single screw machinesxe2x80x9d.
The use of organoclays as thixotropic agents for liquids is also known. Organoclays are used in solvent-borne and water-borne coatings for this reason. They are also used in combination with hydrocarbons to make xe2x80x9cinorganic gelxe2x80x9d greases. The use of organoclays to deliver active ingredients to plastics has been described in European Patent Application Publication No. EP0846660A2 and U.S. Pat. No. 5,721,306. EP 0846660A2 describes the use of clay platelets that have been xe2x80x9cintercalated with long chain monomersxe2x80x9d and xe2x80x9ccombined with a polar organic solventxe2x80x9d to plasticize thermoplastic or thermosetting polymers. No part of this patent application suggests that the inventors recognized the ability to use plasticizers that had been modified with organoclays to eliminate the need for a separate compounding step as provided by the present invention as described below. This patent application also teaches that xe2x80x9cthe exfoliated platelets are free to move throughout the polymer matrixxe2x80x9d and when mixed with a carrier or solvent, xe2x80x9cmaintain viscosity and thixotropy of the carrier materialxe2x80x9d. Such a consideration does not hold for solids at room temperature such as the plasticizers that are the subject of this invention. Finally, the intercalates taught in EP 0846660 are designed to contain at least about 4% water, by weight.
One of the issues faced in the commercialization of highly plasticized thermoplastic formulations is the need to compound the plasticizer with the thermoplastic resin prior to processing the formulation into a finished film, sheet or molded part. This invention describes the use of organoclays as additives to plasticizers to provide a plasticizer composition, to allow the mixing of the plasticizers with the thermoplastic resins in the same process used to convert the resin formulation to finished product. The practice of this invention eliminates the need for a preliminary compounding step prior to extrusion or molding. The preferred plasticizers in this invention are those that are solids at room temperature. Benzoate esters are one class of these solid plasticizers and employed below in the examples. The preferred organoclays in this invention are those based on sodium montmorillonite modified by the exchange of the sodium ion with an organic ammonium ion. In a preferred embodiment, when the plasticizer has sufficient organoclay to have an inorganic content of about 10 wt %, the plasticizer composite can be formed into pellets. These plasticizer pellets can then be blended with thermoplastic resin pellets and the physical blend may then be directly extruded into film or sheet, or can be injection molded into parts.
This invention also provides the use of solid ester plasticizers that have been modified with quaternary ammonium layered smectite clays thereby circumventing the need for a separate compounding step to obtain plasticized thermoplastic products. Thus, the plasticizer that is modified with organoclay and the thermoplastic can be added to the film/sheet extruder or injection-molding machine to make the final product in one step.
As used herein, the terms set forth below will have the following meanings:
xe2x80x9cLayered Materialxe2x80x9d shall mean an inorganic material such as a smectite clay mineral, that is in the form of a plurality of adjacent, bound layers and has a thickness, for each layer of about 3 angstroms to about 50 angstroms, preferably about 10 angstroms.
xe2x80x9cPlateletsxe2x80x9d shall mean individual layers of the Layered Material.
xe2x80x9cGalleryxe2x80x9d shall mean the space between two adjacent Platelets.
A xe2x80x9cclayxe2x80x9d is defined here as a swellable layered clay material, such as the smectite clay mineral montmorillonite. An xe2x80x9corganoclayxe2x80x9d is defined here as a clay that has been ion exchanged with an onium ion.
Thus, in a first embodiment of the present invention, there is provided a plasticizer composition comprising a plasticizer and an organically-modified clay, wherein said composition contains less than 4%, preferably less than 1% water by weight. Moreover, virtually any water which is present will be xe2x80x9cboundxe2x80x9d water. That is, it will exist as part of the crystal structure of the clay and requires temperatures in excess of 250xc2x0 C. to be driven off. The plasticizer compositions of the present invention are useful as processing aids for thermoplastic resins and can be added directly to the thermoplastic resin during the thermal extrusion or molding process to provide plasticized finished articles.
In a second embodiment, the present invention provides a method for blending a plasticizer with a thermoplastic polymer to form a plasticized polymer composition, comprised of about 3 to about 80 weight percent of plasticizer, preferably about 7 to 60 weight percent plasticizer, and most preferably about 10 to about 40 weight percent plasticizer, which comprises
(a) blending at least one plasticizer at or above the melt or softening point temperature of said plasticizer with at least one organically-modified clay to form a plasticizer composition; followed by
(b) melt blending said composition with a thermoplastic polymer.
In the practice of this invention, solid plasticizers are combined with organically modified, layered smectite clays to form plasticizer/clay composites that may be combined with a thermoplastic resin in a film/sheet extruder or injection-molding machine. The modification with the organically modified clay allows the plasticizer to be combined with the thermoplastic resin without having to go through a separate compounding step. There is considerable cost savings from eliminating the capital equipment to perform the compounding step and avoiding the energy requirement for melting the thermoplastic resin in order to combine it with the plasticizer, only to resolidify the plasticized thermoplastic resin and then melt it a second time to extrude the film or mold the product. There is also an increase in product quality because the degradation caused by going through a compounding process at high temperatures is avoided. Thus, in large measure, the molecular weight of the polymer will be preserved and the color formation due to polymer degradation will be avoided. Furthermore, any difficulty in drying a plasticized resin, such as blocking or loss of plasticizer, is avoided when the formulation is converted to the finished product without an intermediate compounding step.
Preferred swellable layered clay materials include natural, synthetic, and modified phyllosilicates. Illustrative of such clays are smectite clays, such as montmorillonite, bentonite, saponite, and hectorite, synthetic clays, such as synthetic hectorite, and modified clays, such as fluoronated montmorillonite. Preferred swellable layered clay materials are phyllosilicates of the 2:1 type having a cation exchange capacity of 50 to 200 milliequivalents per 100 grams of mineral. The most preferred swellable layered clay materials are smectite clay minerals, especially montmorillonite. The organoclay materials of this invention may be prepared by dispersing the clay in hot water, most preferably from 50 to 80xc2x0 C., adding the onium ion with agitation, then blending for a period of time sufficient for the onium compound to exchange most of the cations, usually sodium ions, associated with the layers of the clay. It is desirable to use a sufficient amount of the onium ions to exchange most of the cations present in the galleries. The organoclay material is isolated by methods known in the art, such as filtration, centrifugation, gas-fluidized flake drying bed, freeze drying, or spray drying.
In the context of the present invention, a plasticizer is a substance generally added to a thermoplastic polymer to improve processibility, flexibility and stretchability. Plasticizers in general may decrease melt viscosity, glass transition temperature and the modulus of elasticity of the neat polymer without altering its fundamental chemical character.
Preferred plasticizers include aromatic carboxylic acid esters that are solid at ambient temperature, i.e., about 24xc2x0 C. The formula for such esters is as follows:
ArCOOR
where Ar is an organic aromatic group. The aromatic group may be derivatized further with halogens, hydroxyl, carboxyl, alkyl or aryl groups. These include esters of benzoic acid, naphthanoic acid, phthalic acid, isophthalic acid, terephthalic acid, and naphthalene dicarboxylic acid. The R group may be aryl, alkyl, derivatized aryl or derivatized alkyl groups. The preferred plasticizers are based on esters of benzoic acid. An additional requirement for these plasticizers is that they be compatible with the thermoplastic resin with which they are to be combined at the level they are to be included.
Other examples of suitable plasticizers include the following:
dimethyl isophthalate
diphenyl isophthalate
t-butyl phenyl diphenyl phosphate
triphenyl phosphate
n-butyl acetyl picrinoleate
butyl ricinoleate
glyceryl tri-(acetyl ricinoleate)
methyl acetyl ricinoleate
propylene glycol ricinoleate
n-ethyl p-toluenesulfonamide
p-toluenesulfonamide
dicyclohexyl phthalate
diethylene glycol dibenzoate
dipropylene glycol dibenzoate
polyethylene glycol 200 dibenzoate
neopentyl glycol dibenzoate
cyclohexanedimethanol dibenzoate
glycerol tribenzoate
Preferred onium ions for the process of this invention can be represented as follows: 
wherein R1, R2, R3, and R4 are independently selected from the group consisting of (a) hydrogen; (b) linear or branched alkyl groups having 1 to 22 carbon atoms; (c)aralkyl groups which are benzyl and substituted benzyl moieties including fused ring moieties having linear chains or branches of 1 to 22 carbon atoms in the alkyl portion of the structure; (d) aryl groups such as phenyl and substituted phenyl including fused ring aromatic substituents; and (e) beta, gamma unsaturated groups having six or less carbon atoms or hydroxyalkyl groups having 2 to 6 carbon atoms; and Xxe2x88x92 represents an anion selected from the group consisting of halogen, hydroxide, or acetate anions, preferably chloride and bromide.
Examples of preferred onium ions includes alkyl ammonium ions, such as dodecylammonium, octadecylammonium, dimethyidioctadecylammonium, octadecyltrimethylammonium, benzyldimethyloctadecylammonium, and bis(2-hydroxyethyl)octadecyl methyl ammonium, and the like, and alkyl phosphonium ions, such as octadecyltriphenyl phosphonium. The most preferred ammonium ion for this invention is trimethyloctadecylammonium ion.
The thermoplastic resins referred to herein may be any thermoplastic resins that is conventionally plasticized by a compounding step. The broad classes of such thermoplastic resins include polyesters, polyamides, polycarbonates, cellulose esters and vinyl polymers. Polyesters are the preferred thermoplastic resins. These polyesters cover the range of polyesters made from combinations of the common dicarboxylic acids and diols used in polyesters. Examples of these dicarboxylic acids are terephthalic acid, isophthalic acid, naphthalene dicarboxylic acid, succinic acid, glutaric acid and adipic acid. Examples of common diols are ethylene glycol, 1,3-propanediol, 1,4-butanediol and 1,4-cyclohexane dimethanol. This list is not meant to exclude any particular diacid or diol. The most preferred polyesters are those with at least 50 mole percent of the diacid being terephthalic acid and 50 mole percent of the diol being ethylene glycol.
The organically modified clay may be combined with the plasticizer in a variety of ways. One method, although not preferred, involves the dissolution of the plasticizer in a solvent that will also expand the clay gallery by at least two angstroms allowing the plasticizer to enter the gallery and remain. Further expansion of the gallery by the plasticizer is not necessary although preferred. The solvent is then removed by methods readily practiced in the art such as filtration, centrifugation, gas-fluidized flake drying bed, freeze drying, or spray drying. A more preferred method is melt blending of the plasticizer with the clay. One requirement is that the process of combining the two components be performed above the melting point or softening point of the plasticizer. Many types of heated mixers would be satisfactory for this process, including heated planetary mixers, heated bowl mixers, as well as twin screw extruders. In the case of the twin screw extruder, the extruder produces the plasticizer/clay composite in the form of a strand that may then be easily cut into pellets. These solid pellets may then be blended with pellets of the thermoplastic resin before being fed into the throat of the film/sheet extruder in the practice of the method of the present invention. Alternatively the pellets may be fed simultaneously into the throat of the film/sheet extruder with the pellets of the thermoplastic resin.